HEART By vikas lodhi

Heart

The human heart is a vital organ that functions as a pump, providing a continuous circulation of blood through the body, by way of the cardiac cycles. The heart is contained in the mediastinum in the thoracic cavity of the thorax.

The heart is enclosed in a protective sac, the pericardium which also contains a lubricating pericardial fluid. The outer wall of the heart is made up of three layers, the epicardium, the myocardium which is the muscle of the heart, and the endocardium. The heart is divided into four main chambers: the two upper chambers are called the left atrium and the right atrium (plural atria) and the two lower chambers are called the right and the left ventricle. There is a dividing wall of muscle, called the septum, which separates the right side of the heart from the left side of the heart. The part of the septum that separates the ventricles, the ventricular septum is thicker than that which separates the atria, the atrial septum.

Normally with each heartbeat, the right ventricle pumps the same amount of blood into the lungs that the left ventricle pumps out into the body. Physicians commonly refer to the right atrium and right ventricle together as the right heart and to the left atrium and left ventricle as the left heart.

The human heart and its disorders (cardiovascular diseases) are studied primarily by cardiologists.

Structure

The human heart has a mass of between 250 and 350 grams and is about the size of a large fist.

Chambers

The human heart has four chambers, two superior atria and two inferior ventricles. The atria are the receiving chambers and the ventricles are the discharging chambers.

The pathways of blood through the human heart are part of the pulmonary and systemic circuits. These pathways include the tricuspid valve, the mitral valve, the aortic valve, and the pulmonary valve. The mitral and tricuspid valves are classified as the atrioventricular (AV) valves. This is because they are found between the atria and ventricles. The aortic and pulmonary semi-lunar valves separate the left and right ventricle from the aorta and the pulmonary arterty respectively. These valves are attached to the chordae tendinae (literally the heartstrings), which anchors the valves to the papilla muscles of the heart.

The interatrioventricular septum separates the left atrium and ventricle from the right atrium and ventricle, dividing the heart into two functionally separate and anatomically distinct units.

Walls

It is enclosed in a double-walled protective sac called the pericardium. The two membranes of this sac enclose the pericardial cavity. As with other body cavities the pericardial cavity is lined with two serous membranes, or serosas. The serosas secrete a serous fluid the pericardial fluid which fills the cavity, and acts as a lubricant to prevent friction. This enables the heart to move in response to its own contractions and to the movements of adjacent structures such as the diaphragm and lungs. It also serves as protection from infection and knocks.The outer serous membrane is that of the wall, the parietal membrane, and the serosa closest to the organ is the visceral membrane. Each serosa is made up of a single layer of squamous epithelial tissue as mesothelium which produces the serous fluid. The membranes of mesothelium are tightly bound to an underlying layer of connective tissue which provides the blood vessels and nerves for the overlying secretory cells. On the outer side of the parietal pericardium there is a fibrous layer the fibrous pericardium which is joined to the mediastinal fascia. This fibrous layer anchors the heart's surrounding structures but generally has no effect on heart function.

The outer wall of the human heart is composed of three layers. The outer layer is called the epicardium, or visceral pericardium since it is also the inner wall of the (serous) pericardium. The middle layer of the heart is called the myocardium and is composed of muscle which contracts. The inner layer is called the endocardium and is in contact with the blood that the heart pumps. Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves.

Location

The heart is located in the middle mediastinum, one of the divisions of the thoracic cavity. The heart is at the level of the thoracic vertebrae T5-8. The heart rests on the diaphragm, beneath the sternum and ribs, and has two sides adjacent to the right and left lungs.

The left ventricle, and some of the right ventricle, rest on the central tendon of the diaphragm. At the posterior edge of the surface is the coronary sulcus. 

The side opposite the apex is the base of the heart, and is the most posterior section of the heart. It consists mainly of the left atrium and some of the parts closest to the heart of the right atrium, inferior and superior vena cavae, and pulmonary veins.

The side of the heart facing the sternum and ribs consists mostly of the right ventricle, with some of the right atrium and left ventricle also present.  The coronary sulcus runs down this surface.

Facing the left lung is the left ventricle and some of the left atrium, and facing the right lung is the right ventricle and some of the right atrium.

Function

Blood flows through the heart in one direction, from the atria to the ventricles, and out through the pulmonary artery, and the aorta. Blood is prevented from flowing backwards by the tricuspid, bicuspid, aortic, and pulmonary valves.

The heart acts as a double pump. The function of the right side of the heart is to collect de-oxygenated blood, in the right atrium, from the body (via the superior and inferior venae cavae and pump it, via the right ventricle, into the lungs (pulmonary circulation) where carbon dioxide can be exchanged for oxygen. This happens through the passive process of diffusion.

The left side of the heart collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood flows to the left ventricle which pumps it out to the body (via the aorta).

The lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation. Atria facilitate circulation primarily by allowing uninterrupted venous flow to the heart, preventing the inertia of interrupted venous flow that would otherwise occur at each ventricular systole.[13]

Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle. Here, it is pumped out of the pulmonary semilunar valve and travels through the pulmonary artery to the lungs. From there, blood flows back through the pulmonary vein to the left atrium. It then travels through the mitral valve to the left ventricle, from where it is pumped through the aortic semilunar valve to the aorta and to the rest of the body. The (relatively) deoxygenated blood finally returns to the heart through the superior and inferior venae cavae.

By vikas lodhi 

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